Jansen signed for the American side Washington Diplomats in 1980, but after playing 27 games for them, he returned to the Netherlands to sign for Ajax, on the advice of Johan Cruyff, who played together with Jansen at the Washington Diplomats.

Jansen would play as libero and fill a gap in the Ajax defence after the departure of Ruud Krol in the summer of 1980. By November 1980, after 13 rounds of the Eredivise Ajax were 8th, by the end of the season they had picked up form and finished as runners-up behind AZ and in front of FC Utrecht. The next season Ajax went on to win the Eredivisie with Jansen in the team. Feyenoord fans didn't like former player Jansen playing for their main rivals Ajax. On his Ajax debut (which, incidentally, was away to Feyenoord. Ajax lost 4–2) a snowball was thrown at his eye and Jansen had to leave the field for treatment. Jansen stayed at Ajax until summer 1982 when he retired at age 35.

He began his management career at his old club Feyenoord, where he worked as a coach, and then as assistant manager, between 1983 and 1987. After a season as manager of Belgian club SC Lokeren, he was appointed as Technical Director at second division SVV, where he won the league alongside his manager – future Rangers manager Dick Advocaat.[citation needed]